The present invention generally relates to synchronizing signal generating systems, and more particularly to a synchronizing signal generating system for a laser scanner.
Conventionally, a raster scan type optical recording (or write) apparatus is used in a laser printer, a laser platemaker, a laser facsimile machine, a digital copying machine and the like. The recording of the image information is carried out two-dimensionally by a main scan of a laser beam and a sub scan which is realized by feeding a recording medium.
In such an optical recording apparatus, a recording beam generally modulates an output laser beam of a laser light source depending on the image information and the laser beam is deflected by a deflector such as a polygonal mirror to scan the recording medium such as a photosensitive drum. In order to make an appropriate optical scan, a synchronizing signal is used to synchronize the timing of the optical scan. Generally, a light receiving element is arranged at a position outside a range of the image so as to detect the laser beam prior to each scan, and the synchronizing signal is derived from an output of the light receiving element. However, this method merely synchronizes the scan at the start of the scan. As a result, the scan timing or the scanning speed is not necessarily constant at the end of each scan because it is virtually impossible to maintain the rotation of the polygonal mirror constant and mirror surfaces of the polygonal mirror cannot be finished to perfect mirror surfaces. When the scan timing is not constant at the end of each scan, recording positions of dots become inaccurate and the printing quality deteriorates.
Accordingly, there is a proposal to generate the synchronizing signal using a synchronizing beam and a linear encoder (also referred to as grating, grid or scale) which has a plurality of slits, that the intervals of the recorded dots become constant. This proposed method is disclosed in a U.S. Pat. No. 2,389,403. On the other hand, a Japanese Laid-Open Patent Application No. 54-97050 proposes an improvement which facilitates the production of the linear encoder. According to this proposed method, transparent and non-transparent portions are provided in the linear encoder with a pitch which is N times a recording density, and the synchronizing signal is generated by frequency-dividing by 1/N a pulse signal which is derived from a light which is transmitted through the linear encoder using a phase locked loop (PLL) circuit.
As other methods of generating the synchronizing signal, a Japanese Laid-Open Patent Application No. 60-10967 proposes a method of generating the synchronizing signal using a grating and a Japanese Laid-Open Patent Application No. 60-75168 proposes a method of generating the synchronizing signal using a concave mirror array and a plurality of light receiving elements.
FIG. 1 generally shows an arrangement for carrying out the method proposed in the Japanese Laid-Open Patent Application No. 60-10967. FIG. 1 shows a part of an optical recording apparatus which has a laser scanning optical system using a semiconductor laser as a light source.
In FIG. 1, a semiconductor laser 1 emits a recording beam P1 which is modulated by an image signal. The recording beam P1 is deflected by a mirror surface of a rotating polygonal mirror 2 and passes through an f.theta.-lens 3. The recording beam P1 is then reflected by a mirror 4 and is imaged on a photosensitive body 5. A recording is made on the photosensitive body 5 by a scanning line 6. On the other hand, a synchronizing beam P2 is emitted from a semiconductor laser 7 and impinges the same mirror surface of the polygonal mirror 2 as the recording beam P1. The synchronizing beam P2 hits the mirror surface at a position separated from the position where the recording beam P1 hits this mirror surface. The synchronizing beam P2 passes through the f.theta.-lens 3 similarly to the recording beam P1.
Because the position of the synchronizing beam P2 is different from that of the recording beam P1 along the vertical direction, the synchronizing beam P2 thereafter passes above the mirror 4 and scans a grating 8 which is located at a position which is optically equivalent to that of the photosensitive body 5. The synchronizing beam P2 which is transmitted through transparent portions of the grating 8 is successively imaged on a plurality of (four in this case) light receiving elements 10a through 10d by a lens array 9. A reference pulse signal Pr is derived from outputs of the light receiving elements 10a through 10d. In other words, the outputs of the light receiving elements 10a through 10d are amplified and added. Accordingly, the reference pulse signal Pr becomes a pulse train which exists for the entire scanning region depending on the arrangement of the transparent and non-transparent portions of the grating 8. The reference pulse signal Pr is shaped if needed and is processed in a PLL circuit which generates a synchronizing signal Po.
FIG. 2 shows a conventional PLL circuit. A PLL circuit 11 comprises a voltage controlled oscillator (VCO) 12, a 1/N frequency divider 14, a phase comparator 14, and a lowpass filter 15. The phase comparator 14 compares the reference pulse signal Pr and an output pulse signal Pb of the frequency divider 13. The frequency divider 13 generates the pulse signal Pb by frequency-dividing an output synchronizing signal Po of the VCO 12 by 1/N. An output signal of the phase comparator 14 is indicative of a phase error between the two compared signals, and is supplied to the VCO 12 via the lowpass filter 15 which eliminates unwanted noise and high-frequency components. Hence, a feedback control is made with respect to the VCO 12 so that the phases of the signals Pr and Pb match. Accordingly, the VCO 12 outputs the synchronizing signal Po which is phase synchronized to the reference pulse signal Pr and is multiplied by N. By use of this PLL circuit 11, it becomes possible to generate the synchronizing signal Po which follows a change in the scanning speed, that is, the frequency change in the reference pulse signal Pr.
When the information which is supplied from a printer controller or a host machine to a driving modulation circuit is modulated and recorded in synchronism with the synchronizing signal Po which is obtained from the PLL circuit 11, it is possible to record the information with a highly accurate dot arrangement. In other words, even when the scanning speed changes during the recording due to unstable rotation of the polygonal mirror, the modulation timing of the semiconductor laser is controlled by the synchronizing signal Po so as to ensure an appropriate optical recording.
Depending on the circuit construction and the like of the PLL circuit 11, the is an approximately constant delay in the response of the PLL circuit 11. For this reason, even when the reference pulse signal Pr is generated, the phase of the signals Pr and Pb do not match immediately. For this reason, the Japanese Laid-Open Patent Application No. 54-97050 proposes to provide a gate for the synchronizing signal. The gate is opened to pass the synchronizing signal only after a lock-up time elapses, where the lock-up time is the time required for the phases of the signals Pr and Pb to match. When the laser beam is modulated in synchronism with the synchronizing signal which is obtained according to this method, starting points (that is, first dots) of the effective scanning lines become correctly aligned in the vertical direction. The effective scanning line becomes longer as the lock-up time becomes shorter, thereby making it convenient for obtaining a high resolution. The Japanese Laid-Open Patent Application No. 54-97050 also proposes resetting the frequency divider 13 by a first photoelectric pulse of the scanning line and forcibly matching the phases of the photoelectric pulse signal and the reference pulse signal Pr. By taking this measure, it is possible to reduce the phase correction quantity and reduce the lock-up time.
In the Japanese Laid-Open Patent Application No. 54-97050, the recording beam is emitted from a blue argon laser and the synchronizing beam is emitted from a red helium laser so as to enable composing and separation of the two beams.
When the output synchronizing signal Po of the PLL circuit 11 is used for synchronizing the recording operation of the laser printer, for example, the reference pulse signal Pr which is supplied to the phase comparator 14 becomes discontinuous between a recording region (scan region) and a non-recording region (non-scan region) as shown in FIG. 3. As a result, the output voltage of the lowpass filter 15 which controls the oscillation frequency of the VCO 12 has a large ripple at the time when the reference pulse signal Pr starts and at the time when the reference pulse signal Pr ceases. The PLL stabilizes to a steady state value with a time constant (pull-in time) which is determined by the loop gain of the PLL. The synchronizing signal Po must be output from the PLL circuit 11 after the PLL stabilizes to the steady state value and is locked.
Because of the pull-in time, the recording region shown in FIG. 3 is set wider than the actual recording width. When generating the reference pulse signal Pr shown in FIG. 3 by use of the grating 8, the pull-in time must be made as short as possible so that the size of the apparatus can be reduced. In other words, in this type of PLL, it is important to lock the PLL within a short time.
Therefore, various measures are taken to lock the PLL within a short time.
According to a first method, the time constant of the lowpass filter 15 is switched when inputting the reference pulse signal Pr.
According to a second method, the input voltage of the VCO 12 is maintained to the voltage at the time when the PLL is locked when the reference pulse signal Pr ceases (the non-recording region).
According to a third method, a pseudo pulse signal is used as proposed in a Japanese Laid-Open Patent Application No. 63-234630. When the frequency of the reference pulse signal Pr greatly changes such as the case where the reference pulse signal Pr ceases, the pseudo pulse signal is generated by frequency-dividing an output pulse signal of an oscillator within a pseudo pulse signal generating circuit. The pseudo pulse signal is supplied to the PLL circuit 11 to compensate for the ceased reference pulse signal Pr.
However, in the cases described hereunder, it is impossible to obtain a stable image by use of the synchronizing signal which is generated by the conventional circuit.
For example, according to the method proposed in the Japanese Laid-Open Patent Application No. 54-97050, the frequency dividing ratio is 1/N and constant in the PLL circuit 11, and it is only possible to obtain a fixed recording density. The recording density is L.times.N (dpi) when the pitch of the grating 8 is L (dpi).
On the other hand, Japanese Laid-Open Patent Applications No. 61-66465 and No. 61-66466 propose methods of varying the recording density. The Japanese Laid-Open Patent Application No. 61-66465 varies the recording density by varying the scanning speed of the polygonal mirror and the beam diameter. The Japanese Laid-Open Patent Application No. 61-66466 varies the recording density by varying the synchronizing signal, beam diameter or the feeding speed of the photosensitive body. However, according to these methods, the synchronizing signal is derived from the output of the light receiving element which is arranged at the position outside the range of the image so as to detect the laser beam prior to each scan as described above. As a result, the recording positions of dots become inaccurate and the printing quality deteriorates due to the unstable rotation of the polygonal mirror and the imperfect mirror surfaces of the polygonal mirror. In addition, the Japanese Laid-Open Patent Applications No. 61-66465 and No. 61-66466 also propose to vary the recording density by varying the voltage which is supplied to the VCO 12. But this method results in an unstable synchronizing signal and an unstable image because a voltage change easily occurs due to a temperature change.
According to the method proposed in the Japanese Laid-Open Patent Application No. 54-97050, the dots are recorded at the correct positions only when the temperature is constant. Generally, when an ambient temperature of the PLL circuit changes, a phase error .phi. between the reference pulse signal Pr and the pulse signal Pb changes even when the PLL is locked. For example, even when a phase error .phi..sub.0 exists between the reference pulse signal Pr shown in FIG. 4(A) and the pulse signal Pb shown in FIG. 4(C) at a temperature t.sub.0, the phase error changes to .phi..sub.1 when the temperature changes to t.sub.1. FIG. 4(B) shows a gate signal Pg and FIG. 4(D) shows the pulse signal Pb at the temperature t.sub.1. Therefore, the dots cannot be recorded at the correct positions when the temperature changes, and no measure is taken to guarantee a stable image even when the temperature change occurs.
According to the method proposed in the Japanese Laid-Open Patent Application No. 54-97050, the apparatus becomes bulky because it is necessary to provide a mirror for separating the laser beams emitted from the two light sources and use two gas lasers for the light sources. It is conceivable to use semiconductor lasers with infrared wavelengths as the light sources of the recording beam and the synchronizing beam as proposed in the Japanese Laid-Open Patent Application No. 60-10967, so as to reduce the size of the apparatus. But although the recording beam is modulated depending on the image signal, the synchronizing beam is constantly emitted from the light source after the power source of the laser printer or the like is turned ON so that it is possible to constantly generate the synchronizing signal based on the synchronizing beam. For this reason, when the standby time is long, the photosensitive body, the film and the like are exposed by the flare of the synchronizing beam and makes it impossible to obtain a stable image.
Furthermore, with regard to the measures taken to lock the PLL within a short time, a ringing occurs when the time constant of the lowpass filter is switched according to the first method. The second method is easily affected by the temperature change. In addition, according to the third method, a noise which is generated by the oscillator within the pseudo signal generating circuit affects the PLL even when the reference pulse signal is originally generated, and makes the PLL operation unstable. This results in an unstable image.